1. Field of the Invention
The present invention relates generally to an automatic gain control (AGC) apparatus and method in a communication system. In particular, the present invention relates to an apparatus and method for performing AGC to maintain received signal power uniformly in an Orthogonal Frequency Division Multiple Access (OFDMA) wireless communication system.
2. Description of the Related Art
A conventional wireless communication system is typically a voice service-oriented mobile communication system. With the development of communication industry and the increasing user demand for Internet service, the wireless communication system is evolving into an advanced communication system capable of supporting high-speed data transmission. However, the wireless communication system, originally developed to mainly provide the voice service, has reached a limit in transmitting a large volume of data due to the exhaustion of data transmission bandwidth and resource. Therefore, conventional Code Division Multiple Access (CDMA) mobile communication system cannot fully meet the demands in the modern society requiring a system capable of transmitting large volume of data as well as voice at high speed.
One of the methods proposed to solve the problems is Orthogonal Frequency Division Multiplexing (OFDM). A description will now be made of the OFDM technology.
The OFDM technology, a scheme for transmitting data using multiple carriers, is a kind of Multi-Carrier Modulation (MCM) that converts a serial input symbol stream into parallel symbols and modulates each of the parallel symbols with a plurality of orthogonal sub-carriers before transmission. The OFDM technology has the merit of maximizing throughput using a link adaptation algorithm as well as the merit of being robust against frequency selective fading.
Meanwhile, OFDMA, an OFDM-based multiple access scheme, reconfigures some of all possible sub-carriers into a sub-channel and allocates the sub-channel to a particular subscriber station (SS).
The term “sub-channel” as used herein refers to a channel comprised of at least one sub-carrier. The use of OFDMA enables dynamic resource allocation that can dynamically allocate a sub-channel a particular SS according to a fading characteristic of a wireless channel, and an increase in the number of SSs, meaning users, increases multiuser diversity gain.
FIG. 1 is a block diagram illustrating a configuration of a physical layer for data transmission/reception in a general OFDMA system. In FIG. 1, reference numerals 102 through 107 denote a structure of a transmitter, and reference numerals 121 through 127 denote a structure of a receiver.
A transmission input bit stream 101 is input to an encoder 102. The encoder 102 encodes the input bit stream 101 according to a predetermined coding scheme and outputs the coded serial input bit stream to a serial-to-parallel (S/P) converter 103. The S/P converter 103 converts the coded serial input bit stream into a parallel bit stream in order to perform inverse fast Fourier transform (IFFT), and provides the parallel bit stream to an IFFT unit 104. It is assumed herein that the parallel bit stream includes N symbols. The reason for assuming that the IFFT unit 104 receives N-symbol bit stream is because the IFFT unit 104 performs IFFT on the input bit stream in units of N symbols.
The IFFT unit 104 performs IFFT on N transmission symbols received in parallel to convert frequency-domain symbols into time-domain symbols, and outputs the time-domain symbols to a parallel-to-serial (P/S) converter 105. The P/S converter 105 converts the N time-domain symbols received in parallel into a serial stream of N sequential bits. Herein, the stream of N sequential bits will be referred to as an “OFDM symbol.”
The OFDM symbol is input to a cyclic prefix (CP) adder 106. The CP adder 106 copies a predetermined number of last bits in the input OFDM symbol, and inserts the copied bits in front of the initial bit of the OFDM symbol, in order to remove an influence of a multipath channel. The CP-added OFDM symbol is input to a digital-to-analog (D/A) converter 107. The D/A converter 107 converts the input digital symbols into analog symbols and transmits the analog symbols to a receiver.
The transmitted analog symbols are received at the receiver via a channel 110 having multiple paths. A description will now be made of a structure and operation of the receiver.
An analog-to-digital (A/D) converter 121 converts the analog signal received over the channel 110 into a digital signal, and outputs the digital signal to a CP remover 122. The CP remover 122 removes CPs contaminated due to the influence of the multiple paths, from the input digital signal, and outputs the CP-removed serial signal to an S/P converter 123, for fast Fourier transform (FFT) unit. The S/P converter 123 parallel-converts the symbols received in series in units of N symbols because the transmitter performed IFFT in units of N symbols.
An FFT unit 124 receives N-symbol parallel data and performs FFT on the received signal to convert time-domain symbols into frequency-domain symbols. The frequency-domain symbols are input to an equalizer 125. The equalizer 125 cancels an influence of the channel 110 from the frequency-domain symbols, and provides its output symbols to a P/S converter 126. The P/S converter 126 converts the parallel input symbols back into serial symbols in units of N symbols, and outputs the serial symbols to a decoder 127 in units of N symbols. The decoder 127 decodes the input symbols into an output bit stream 128.
FIG. 2 is a diagram illustrating an exemplary internal structure of a general AGC circuit. As illustrated in FIG. 2, in the process of designing an OFDMA transceiver, an AGC loop for maintaining received signal strength uniformly should be provided in front of the A/D converter 121 because of time variation of a channel. The AGC circuit includes an A/D converter (ADC) 210, a power measurer 220, an accumulator #1 230, a loop filter 240, a log MAP table 250, an accumulator #2 260, an AGC interface 270, and a variable gain amplifier 200.
In the conventional AGC loop, as shown in FIG. 2, the power measurer 220 receives signals sampled on the time axis from the ADC 210, and accumulates norms of the sampled signals to measure average power for a predetermined time. The accumulator #1 230 accumulates the average power measured by the power measurer 220 for a predetermined time, and the loop filter 240 loop-filters the accumulated average power. The log MAP table 250 compares the average power output from the loop filter 240 with predetermined reference power for the ADC input, and outputs the resultant comparison value to the accumulator #2 260. The accumulator #2 260 continues to accumulate the comparison value, and outputs the accumulated value to the AGC interface 270. The AGC interface 270 controls a gain of the variable gain amplifier 200 in the analog stage using the accumulated comparison value in such a manner that it provides a digital value obtained from the accumulated comparison value to the variable gain amplifier 200.
With reference to FIG. 3, a description will now be made of a structure of a frame commonly transmitted/received in an OFDMA wireless communication system. FIG. 3 is a diagram illustrating a structure of a general OFDMA frame.
Referring to FIG. 3, an OFDMA frame includes both uplink (UL) information and downlink (DL) information therein, and also includes Transmit Time Gap (TTG) information and Receive Time Gap (RTG) information between the uplink and downlink information. In FIG. 3, the OFDMA frame includes a DL sub-frame for the downlink in its left-hand side and includes a UL sub-frame for the uplink in its right-hand side, and each frame includes a preamble 301 and includes a DL-MAP 302 and a UL-MAP 303 as MAP information.
The DL/UL-MAPs 302 and 303 each divide DL/UL sub-frames for the downlink and the uplink into several intervals and allocate thereto location information of each interval, and a connection identifier (CID) and a Downlink Interval Usage Code (DIUC)/Uplink Interval Usage Code (UIUC) for each interval. Herein, the CID, which is a subscriber identification code, indicates data for which SS the corresponding interval transmits, and the DIUC/UIUC, which is a value indicating usage, modulation type and frame error control (FEC) code, indicates usage of data for the corresponding interval, a modulation type in which the corresponding data is modulated, and an FEC code with which the corresponding data is encoded.
In OFDMA, an allocated sub-channel transmits data along with a pilot signal on a predetermined power level, but a non-allocated sub-channel transmits only the pilot signal and transmission power at a transmission unit differs according to sub-channel allocation information. For example, Partial Usage of Sub-Channels (PUSC) defined in IEEE 802.16e differs in power up to a maximum of 6.4 dB according to sub-channel allocation content. As illustrated in FIG. 3, for one OFDMA frame defined in IEEE 802.16e, if powers of ith, jth and kth symbols are denoted by Psym,i, Psym,j, Psym,k, respectively, then power levels of the symbols differ from each other in order of Psym,i>Psym,j>Psym,k according to the sub-channel allocation content.
As described above, an OFDMA system has a wireless communication environment in which transmission power differs according to the sub-channel allocation content. When a signal is received over a channel, a variation in received power on the time axis includes not only a channel variation but also a difference in transmission power. However, in order to restore information carried in a signal level in the frequency domain, the AGC loop should operate such that it compensates only for the variation in channel. To this end, the difference in transmission power should be normalized so that it is not reflected in the AGC loop.
In this context, in the OFDM wireless communication system, because average power at a transmission unit is probably constant during data transmission, a difference between average power calculated on the time axis at a reception unit and reference power reflects a change in channel. However, in the OFDMA system, because transmission power from a transmission unit is not constant on average according to the sub-channel allocation content, average power calculated at a reception unit in the manner of FIG. 2 reflects not only the channel variation but also the difference in transmission power undesirably. This problem affects the process of restoring information carried in the signal level in the frequency domain at the reception unit.